1. Field of the Invention
The present invention relates to a thermal transfer printer of the intermediate transfer type that produces a desired record by melting ink from an ink ribbon by heat of a thermal head, then transferring the molten ink onto an intermediate transfer medium so as to form a primary recording image, and finally re-transferring, namely, transferring this primary recording image onto paper and to a printing method therefor.
2. Description of the Related Art
Hitherto, there has been frequently used a thermal transfer printer which produces a record by pressing a thermal head against a platen through paper and an ink ribbon. In the case where such a thermal transfer printer produces a record on paper having large surface roughness, such as bond paper, it is necessary to press the thermal head against the paper put on the platen by applying large pressing force to the thermal head. However, there is a limit to the pressing force, because of the precise structure of the thermal head. Thus, there has been used a thermal transfer printer of the intermediate transfer type that produces a record by partly melting ink from an ink ribbon by heat generated in the thermal head, then transferring the molten ink onto an intermediate transfer medium so as to form a primary recording image on the primary transfer medium, and finally transferring this primary recording image onto paper pressed against the primary transfer medium by the large pressing force of a pressure roller once more.
FIG. 10 shows a part of the configuration of an example of the conventional thermal transfer printer of such a type. As shown in this figure, a platen roller 61 formed like a metallic cylinder, the surface of which is covered with a rubber material serving as an intermediate transfer medium 60, is provided in this printer in such a manner as to be able to be driven rotatably. Thermal head 62 is provided in the vicinity of the platen roller 61. This thermal head 62 has a plurality of heating elements aligned in such a manner as to face the central portion of the platen roller 61. A pair of ribbon rolls 65 and 65a, between which an ink ribbon 64 wound around a bobbin 63 is guided in a nearly linear manner through the platen roller 61 and the thermal head 62, are provided on both sides of this thermal head 62, respectively. Further, a ribbon feeding mechanism is provided halfway between the thermal head 62 and the take-up ribbon roll 65a. The ribbon feeding mechanism consists of a pinch roller 66, a ribbon guide shaft 67 for guiding the ink ribbon 64 in such a way as to be fed around the pinch roller 66, and a motor (not shown) for driving the pinch roller 66 to rotate. The speed of feeding the ink ribbon is controlled by driving the pinch roller 66 in such a manner as to become constant.
Moreover, a pressure roller 68 to be pressed against the platen roller 61 under large pressing force or pressure is rotatably provided at a position which is symmetric to the position of the thermal head 62 with respect to the center of the circular transverse section of the platen roller 61. A predetermined kind of paper 69 such as plain paper is inserted into a gap between the platen roller 61 and the pressure roller 68. The paper is put between the platen roller 61 and the pressure roller 68 and is further fed in the direction opposite to a direction, in which the ink ribbon 64 is fed, by driving the platen roller 61 to rotate.
In the case of this thermal transfer printer, when the paper 69 is inserted into the gap between the platen roller 61 and the pressure roller 68, the pressure roller 68 is first pressed against the platen roller 61 under large pressing force or pressure. Further, while the printer is in such a state, the pinch roller 66 is driven to rotate so as to feed the ink ribbon 64 at a constant speed. Moreover, while taking up the ink ribbon 64 by driving a take-up bobbin 63 to rotate, each of the heating elements is caused to generate heat according to a desired recording signal. Then, the paper 69 is fed by driving the platen roller 61 to rotate. Furthermore, part of ink from the ink ribbon 64 is melted by selectively causing the heating elements of the thermal head 62 to generate heat. Then, the molten ink is transferred onto the surface of the platen roller 61. Subsequently, a first recording image formed on the surface of the intermediate transfer medium 60 in this way is re-transferred onto the fed paper 69 by the pressing force of the pressure roller 68. Consequently, a desired record is obtained.
Meanwhile, the pinch roller 66 is provided in the printer for the following purpose.
Namely, regarding the feeding of the ink ribbon 64, in the case where a rubber material, of which the smoothness (or surface roughness) in terms of R.sub.max is not less than 50 .mu.m and the hardness is not less than 60 degrees, is used as the intermediate transfer medium 60, the adhesion of which is represented in terms of the smoothness and the hardness (what is called JIS (Japanese Industrial Standards) hardness) when feeding the ink ribbon 64 by driving the platen roller 61 pressed against the ink ribbon 64 to rotate, a slippage occurs between the platen roller 61 and the ink ribbon 64 and thus the operation of feeding the ink ribbon can not be achieved well if a printing is performed on condition that the head pressing force P of the thermal head 62 is 100 to 300 g/cm.sup.2. Further, in the case where the ink ribbon 64 is fed by driving the take-up ribbon roll 65a to rotate, the speed of the fed ink ribbon 64 changes according to the outside diameter of the take-up ribbon roll 65a and thus it can not be expected that a uniform resultant record is obtained. Therefore, the pinch roller 66 is provided in the printer as a means for controlling the speed of the fed ink ribbon 64 in such a manner that the ink ribbon 64 is stably fed at a constant speed.
Further, when designing the mechanisms of the printer of this type, for the purpose of preventing the fed ink ribbon from becoming loose, such an ink-ribbon feeding speed is set in such a manner that a speed V.sub.1, at which the ink ribbon 64 is fed by rotating the pinch roller, is a little higher than a speed V.sub.0 at which the ink ribbon 64 is fed by rotating the platen roller.
The precision of writing or printing is, however, determined according to the rotating speed of the platen roller 61. Even in the case where the ink-ribbon feeding speed is adjusted to the writing speed on the basis of calculation, the thickness of the transferred ink can not be made to be constant owing to the subtle difference in rotating speed between the platen roller and the pinch roller if the outside diameters of the platen roller 61 and the pinch roller 66 vary even in small lengths within the accuracies of such components.
Further, if the speed V.sub.1 is higher than the speed V.sub.0, slippage occurs between the ink ribbon 64 and the platen roller 61 in the case of using the intermediate transfer medium 60, of which the R.sub.max is not less than 50 .mu.m and the hardness is not less than 60 degrees, as described above. A result of an experiment proves that slippage occurs if the coefficient of friction for the intermediate transfer medium 60, which may change according to the kinds of rubber materials applied to the surface 7 and the platen roller 61, is about 0.95 or less.
Moreover, components associated with the pinch roller 66, as well as a driving-force source therefor, become necessary. Consequently, the cost of the printer becomes high. In addition, the miniaturization of the thermal transfer printer can not be achieved.
Further, FIG. 11 shows a sectional view of a part of another conventional thermal transfer printer of the intermediate transfer type. In this thermal transfer printer, a cylindrical platen roller 73 is provided in such a manner as to face heating elements of a thermal head 71 through an ink ribbon 72. In this platen roller 73, a first heater 75 for heating an intermediate transfer medium 74 through the platen roller 73 is provided. Moreover, a cylindrical back-up roller 78 is provided therein in such a manner as to face a pressure roller 77, which is adapted to be able to advance and retreat in the directions of arrows B1 and B2, respectively, so as to press printing paper 76 against the intermediate transfer medium 74, through the printing paper 76. In this back-up roller 78, a second heater 79 for heating the intermediate transfer medium 74 through the back-up roller 78 is provided. Further, the intermediate transfer medium 74 is looped around the platen roller 73 and the back-up roller 78 under tension. The intermediate transfer medium 74 and the paper 76 are inserted between the pressure roller 77 and the back-up roller 78.
Hereupon, the thermal head 71 advances and retreats in the directions of arrows A1 and A2, respectively, and thus comes into contact with the intermediate transfer medium 74 through the ink ribbon 72. A position on the intermediate transfer medium 74, onto which an ink layer 80 of the ink ribbon 72 is primarily or initially transferred, is the midpoint position of a portion of the medium fed around a part of the platen roller 73 for heating the intermediate transfer medium 74, which corresponds to an arc L1 illustrated in FIG. 12.
Further, the platen roller 73 and the back-up roller 78 are made of metallic materials and have hollow structures so as to contain the first heater 75 and the second heater 79, respectively. Moreover, the platen roller 73 is connected to a driving means (not shown) and is driven by the driving means to rotate clockwise. As the result of rotating the platen roller 73, the intermediate transfer medium 74 is made to run in the direction of an arrow C. Consequently, the ink ribbon 72 and the paper 76 are made to run in the directions of arrows D and E, respectively.
An operation of the conventional thermal transfer printer with such a configuration will be described hereinbelow. First, the thermal head 71 is moved in the direction of the arrow Al and is pressed against the platen roller 73 through the ink ribbon 72 and the intermediate transfer medium 74. At that time, the platen roller 73 is heated by the first heater 75 contained therein. The intermediate transfer medium 74 is heated through this roller to a temperature T1 at which the ink layer 80 of the ink ribbon 72 does not melt. Then, when a plurality of heating elements of the thermal head 71 are selectively heated, the ink layer 80 of the ink ribbon 72 is transferred onto the intermediate transfer medium as a primary recording image.
Thereafter, when the platen roller 73 is driven by a driving-power source (not shown) to rotate, the intermediate transfer medium 74 looped around the platen roller 73 and the back-up roller 78 under tension is fed in the direction of the arrow C. Simultaneously with this, the ink ribbon 72 is driven by friction between the intermediate transfer medium 74 and this ribbon and is thus fed at a constant speed in the direction of the arrow D.
Subsequently, the ink ribbon 72 is taken up by driving a take-up ribbon roll 82 after the primary transfer of the ink layer 80 onto this ribbon has been finished. At that time, a change in the take-up speed depends on the take-up radius of the ribbon roll 82. Therefore, the take-up speed should be higher than the speed at which the intermediate transfer medium is fed. The change in the take-up speed is, however, accommodated by providing a slipping mechanism (not shown) in a take-up mechanism of the ribbon roll 82.
Further, when the intermediate transfer medium, onto which the ink layer 80 has been primarily transferred as the primary recording image, is fed to a place where this ink layer 80 faces the gap between the pressure roller 77 and the back-up roller 78, the pressure roller 77 presses the printing paper 76 against the primary recording image transferred onto the intermediate transfer medium 74. Then, the intermediate transfer medium 74 is heated by the second heater 79 through the back-up roller 78 to an optimum temperature T2 for re-transferring the primary recording image onto the paper. Moreover, the intermediate transfer medium 74 and the paper 76 are fed in the direction of the arrow E. Thereby, the primary recording image having been transferred onto the intermediate transfer medium 74 is re-transferred onto the paper 76. Thereafter, when the pressure roller 77 is moved in the direction of the arrow B2, the pressure exerted upon the intermediate transfer medium 74 is eliminated. Consequently, the operation of printing image is completed. The printer is adapted to record images by performing the foregoing process repeatedly.
Meanwhile, in the conventional thermal transfer printer, the position, at which the primary recording image is transferred onto the intermediate transfer medium 74 by the thermal head 71, is determined without fully considering a position at which such an image should be primarily transferred thereto. Generally, a place as indicated by a reference character b in FIG. 12 in the middle section of a portion of the intermediate transfer medium 74, which is heated when the medium is being fed around a part of the platen roller 73 corresponding to an arc L1, is often determined as a position at which the primary recording image should be primarily transferred thereto.
However, in the case where the intermediate transfer medium is fed around the platen roller 73 which rotates, such a conventionally determined portion of this medium, which corresponds to the position of the thermal head 71, may not be fully heated by the first heater 75 of the platen roller 73. Namely, sometimes, the temperature of such a portion does not reach an appropriate level at which the ink layer 80 is transferred to the intermediate transfer medium 74.
In other words, because the intermediate transfer medium 74 is not heated by the first heater 75 until the medium 74 is fed to a position at which the medium 74 comes in contact with the platen roller 73, the temperature of the medium 74 is relatively low. After the intermediate transfer medium 74 comes in contact with the platen roller 73, this medium is gradually heated by the platen roller 73 while being fed around the platen roller 73. Consequently, the intermediate transfer medium 74 comes to be heated to a temperature suitable for forming the predetermined primary recording image.
However, at the position b of the conventional thermal head 71, where the ink layer 80 is transferred, the intermediate transfer medium 74 is insufficiently heated because of the fact that the distance between the position b and a position on the platen roller 73, at which the intermediate transfer medium 74 comes in contact therewith. Thus the temperature of the intermediate transfer medium 74 sometimes does not reach the range of appropriate temperature. In such a case, the primary recording image is unreliably transferred onto the intermediate transfer medium 74. Thus, there is a fear that the stable reproducibility of dots cannot be obtained.
Moreover, in the case where a color printing is performed by using such a conventional thermal transfer printer, the color printing is carried out by using, for example, a color ink sheet which is called a multicolor ink sheet containing ink of four colors such as yellow (Y), magenta (M), cyan (C) and black (Bk). Namely, one page (or screen) of an image is first printed on a recording medium by using yellow ink of the color ink sheet. Subsequently, the recording medium having been fed during this printing operation is fed back so as to locate a printing position therein. Furthermore, the beginning of a band or area of magenta (M) ink is located in the color ink sheet. Then, one page (or screen) of the image is printed on the recording medium by using the magenta (M) ink. Thereafter, the image is printed on the recording medium by using cyan (C) ink and black (Bk) ink in this order. In this way, the color image is printed on the recording medium.
However, in the case where a color printing is performed by using the aforesaid thermal transfer printer, the formation of a primary recording image on an intermediate transfer member or medium, as well as the transfer and fixing of the primary recording image, is performed correspondingly to each color. Further, in the case where an image or character is printed in ink of a desired color obtained by overprinting the image or character in ink of a plurality of colors (for instance, the image or character is recorded in orange ink by printing the image in yellow (Y) ink over the image in magenta (M) ink), ink of a color is overcoated with ink of another color. This results in reduction in the smoothness of the surface of the overcoated ink (namely, this results in the uneven surface of the overcoated ink). Further, the conventional thermal transfer printer has problems or drawbacks in that the abrasion resistance (namely, the resistance of ink to peeling at the time of rubbing the surface of a print) is low, that thus the degree of fixation of ink onto a recording medium becomes low and that good printing quality cannot be obtained.
Moreover, the conventional thermal transfer printer has problems or drawbacks in that the reduction in the smoothness of the surface of ink ends in decrease in the abrasion resistance of a recording medium having large surface roughness, for example, what is called Lancaster bond paper, that thus good printing quality can not be obtained, that the reduction in the smoothness of the surface of ink also results in decrease in the light transmittance of over-head projector (OHP) paper (namely, transparent paper) and that when an image printed on the OHP paper is projected on a screen by using an OHP, the projected image becomes somber and thus a clear color tone can not be obtained.
The present invention is accomplished to resolve the aforementioned problems of the conventional thermal transfer printer.